1. Field of the Invention
The present invention relates a plasma processing apparatus for performing plasma processing such as cleaning of foreign matter, for example, organic substance existing on a surface of an article, separation of a resist, etching of an organic film, reduction of a metallic oxide, a film formation and surface modification and a plasma processing method employing the plasma processing apparatus. The plasma processing apparatus and the plasma processing method are applicable to cleaning of a surface of an electronic component.
2. Description of the Prior Art
Conventionally, an attempt to perform plasma processing under atmospheric pressure has been made. For example, Japanese Patent Laid-Open Publication Nos. 2-15171 (1990), 3-241739 (1991) and 1-306569 (1989) disclose a plasma processing method in which a pair of electrodes are disposed in a discharge space in a reaction vessel and a dielectric member is provided between the electrodes such that the discharge space is filled with plasma producing gas mainly consisting of rare gas such as helium and argon. An article to be processed is inserted into the reaction vessel. When an AC voltage is applied between the electrodes, glow discharge is induced so as to produce plasma in the reaction vessel through excitation of the plasma producing gas such that the article is subjected to plasma processing by the plasma.
However, this known plasma processing method has such drawbacks that it is difficult to perform plasma processing of a specific portion of the article and a long time is required for plasma processing. Thus, by developing the known plasma processing method, plasma processing in which plasma produced under atmospheric pressure by glow discharge, especially active species of the plasma is jettingly blown to an article to be processed is proposed in, for example, Japanese Patent Laid-Open Publication Nos. 4-358076 (1992), 3-219082 (1991), 4-212253 (1992) and 6-108257 (1994).
In the above mentioned prior art plasma processing method employing the plasma jet, if frequency of AC in use is less than 10 kHz, quantity of generated radicals is reduced and thus, effects of plasma processing are lessened. Therefore, in the prior art processing method, since AC having a frequency of 10 to several tens MHz is used, such a problem arises that the article is oxidized or carbonized due to extreme rise of temperature of the plasma gas, thereby resulting in thermal damage to the article.
Plasma is heated by repetition of collision of gas particles through discharge. Since average free path of gas particles in plasma induced under atmospheric pressure is smaller than that in plasma induced under reduced pressure, frequency of collision of gas particles in the plasma induced under atmospheric pressure becomes higher than that in the plasma induced under reduced pressure, so that temperature of the plasma induced under atmospheric pressure rises extremely. Meanwhile, frequency of collision of gas particles in plasma is closely dependent also on frequency of applied AC and increases further as frequency of applied AC becomes higher, thereby resulting in extremely high temperature of the plasma in a reaction region. Therefore, if frequency of applied AC becomes higher, quantity of radicals and ions in the plasma increases further but temperature of a reaction pipe and electrodes, especially an electrode in the reaction pipe, whose heat is less likely to be dissipated is raised by the high-temperature plasma, so that heat of the electrode in the reaction pipe is, in turn, conducted to the plasma and thus, temperature of the plasma is raised excessively. Moreover, such a disadvantage is incurred that temperature of the electrode in the reaction pipe approximately reaches its melting point according to kinds of metal of the electrode in the reaction pipe and thus, the electrode in the reaction pipe is corroded and volatilized so as to be deposited on the article.
In addition, in the above mentioned prior art plasma processing method employing the plasma jet, such an inconvenience is incurred that streamer discharge is readily induced towards the article so as to damage a surface of the article.
Process of induction of streamer discharge is described below with reference to FIGS. 24 and 25 showing a conventional plasma processing apparatus. The conventional plasma processing apparatus includes a tubular reaction pipe 2, an outer electrode 1 fitted around the reaction pipe 2 and a central electrode 3 disposed in the reaction pipe 2. When not only plasma producing gas is introduced into the reaction pipe 2 but an AC voltage is applied between the outer electrode 1 and the central electrode 3, glow discharge is generated in the reaction pipe 2 so as to produce plasma in the reaction pipe 2 through excitation of the plasma producing gas. The plasma is blown, as a plasma jet, from a blow-off outlet 21 of the reaction pipe 2 to an article 7 such that the article 7 is subjected to plasma processing.
In this conventional plasma processing apparatus, one cause of induction of a streamer discharge 170 of FIG. 25 is that temperature of inside of the reaction pipe 2 and the outer electrode 1 is raised by the high-temperature plasma mainly through heat transfer. Namely, since local temperature rise of a space between the outer electrode 1 and the central electrode 3, i.e. a discharge space 22 causes partial emission of electrons, electrons flow so as to be concentrated at the portion. Another cause of induction of the streamer discharge 170 is that the argon in the plasma producing gas is high in concentration. Namely, since diffusion rate and thermal conductivity of helium are high, helium mitigates local temperature rise. On the other hand, diffusion rate and thermal conductivity of argon are low. Therefore, as argon in the plasma producing gas becomes higher in concentration, heat dissipation property in space deteriorates further.
More specifically, electrons are initially concentrated at portions of surfaces of the reaction pipe 2 and the central electrode 3. Then, streamers 167 are induced between these portions of the surfaces of the reaction pipe 2 and the central pipe 3 as shown in FIG. 24. Once such streamers 167 are induced, stable glow discharge, i.e. stable plasma is no longer induced and thus, it is impossible to perform plasma processing satisfactorily. Furthermore, the streamer discharge 170 is induced not only between the outer electrode 1 and the central electrode 3 but between the central electrode 3 and the article 7 as shown in FIG. 25 so as to damage a surface of the article 7.
In the above mentioned prior art plasma processing method in which plasma processing is performed under atmospheric pressure by employing the plasma jet, if frequency of AC is raised so as to enhance processing effects, the article 7 sustains thermal damage or damage due to the streamer discharge 170. However, if frequency of AC is lowered in order to prevent such damages to the article 7, processing effects are lessened. These disadvantages constitute a serious obstacle to plasma processing of the article 7, for example, various materials including thermally weak organic substance, readily oxidizable metals such as silver and copper and readily fusible solder as well as an electronic component including an IC chip.
Accordingly, an essential object of the present invention is to provide, with a view to eliminating the above described drawbacks of prior art, a plasma processing apparatus and a plasma processing method employing the plasma processing apparatus, which are capable of performing plasma processing of an article by lessening thermal damage to the article and damage to the article caused by streamer discharge.
In order to accomplish this object of the present invention, a plasma processing apparatus for performing plasma processing of an article, according to the present invention comprises: a central electrode; a tubular outer electrode which is provided so as to surround the central electrode; a tubular reaction pipe which is disposed between the central electrode and the outer electrode so as to electrically insulate the central electrode and the outer electrode from each other; a gas supply means for supplying a plasma producing gas to a discharge space defined between the central electrode and the outer electrode in the reaction pipe; an AC power source for applying an AC voltage between the central electrode and the outer electrode; wherein not only the plasma producing gas is supplied to the discharge space by the gas supply means but the AC voltage is applied between the central electrode and the outer electrode by the AC power source so as to generate a glow discharge in the discharge space under atmospheric pressure such that a plasma jet is blown to the article from a blow-off outlet of the reaction pipe; and a cooling means for cooling the central electrode and the outer electrode.